The present invention relates generally to subcarrier structures and semiconductor devices. More particularly but not exclusively, this invention relates to subcarrier structures for use in immovably mounting a variety of types of semiconductor elements including but not limited to semiconductor lasers, which are preferably adapted to be built in Peltier cooler-associated optical semiconductor modules. The invention also relates to semiconductor devices employing such subcarriers.
Electronic component support structures or packages called xe2x80x9csubcarriersxe2x80x9d are typically employed to immovably mount a wide variety of types of semiconductor elements including semiconductor lasers and transistors or else. The subcarriers are required to make use of a certain material with preselected thermal expansion coefficient for suppression or xe2x80x9cmoderationxe2x80x9d of possible thermal distortion in view of longevity of a semiconductor element used. It is also required that the material be excellent in thermal conductivity to facilitate or xe2x80x9cacceleratexe2x80x9d outward escape of heat radiated from semiconductor elements toward an outer housing or package enclosure with increased efficiency. The subcarriers include those using optical semiconductor elements, such as for example semiconductor lasers or light-emitting diodes (LEDs) or else, some examples of which will be described below.
Most optical semiconductor elements are variable in characteristics with a change in temperature. Accordingly, in those optical conductor modules as required to offer constant characteristics, a subcarrier that mounts one or more semiconductor elements as used therein is likewise required to let these elements remain constant in temperature during operations. To do this, the subcarrier is designed to employ cooling means. In case the optical semiconductor elements are in high-speed modulation modes, it will also be required that electrical lead wiring be done by use of a specific strip line pattern which has a prespecified impedance value and is formed to extend up to xe2x80x9cnearbyxe2x80x9d portions of such optical semiconductor elements.
During such modulation operations the subcarrier can experience creation of electrical capacitance between the Earth or ground plane of a optical semiconductor element on the subcarrier and the underlying Peltier cooler, which would affect modulation signals. The higher the signal transmission frequency, the greater the influence. Thus, at higher transfer frequencies, the subcarrier should be arranged so that the capacitance stays less in value. One prior known approach to reducing the ground-to-cooler capacitance is to increase the thickness of the subcarrier as rigidly mounted on the Peltier cooler.
Another problem faced with the related art subcarrier structure lies in its limited cooling/heating abilities. This would result in limitation of temperature differences between the optical semiconductor element and its outside atmosphere. To achieve increased temperature differences therebetween, it is required to minimize heat radiation and absorption or thermal exchange relative to the subcarrier held on the Peltier cooler. This also leads to improvements in efficiency.
See Fig 7. This figure of drawing illustrates, in cross-section, one related art modulator-associated optical semiconductor laser diode module, also known as electroabsorption modulator laser (EML) module among those skilled in the art to which the invention pertains. The module contains a subcarrier as mounted therein. At part (a) of this drawing, there is depicted a planar sectional view of the EML module whereas part (b) shows its side view in cross-section. In FIGS. 7(a)-(b), reference numeral xe2x80x9c71xe2x80x9d designates an EML chip; numeral 72 denotes a subcarrier for rigid attachment of the EML chip; 73 indicates a thermistor for detection of the temperature of the EML chip; 74 shows a photodiode (PD) chip for detection of the amount of light intensity as emitted from the EML chip; 75 is a sub-mount member for fixation of the PD; 76, a glass window as immovably attached to a PKG 81 for permitting outward radiation of light from the EML chip; 78, a metal plate for tightly jointing said lens holder and subcarrier together; 79, a lens for collection of light from the EML chip; 80, a lens holder made of a metallic material for fixation of said lens 79; 81, a package (PKG) of the EML module; 82, a Peltier cooler for temperature control of the EML chip; 83, electrical leads of the PKG 81; 84, bonding wires of gold (Au) for electrical interconnection with the leads 83; 85, a terminate end resistor at which more than one terminate-end resistor of the modulator in the EML chip is formed; 87, an isolator for use in preventing production of return light from the outside; 88, a coupling lens for introducing light emitted from the EML into an optical fiber; 89, optical fiber; 90, ferrule holder for securing the optical fiber 89 to isolator 87; and, 91, link plate (bridge) as formed in the PKG for fixation of the PD sub-mount.
The EML chip 71 is mounted with its junction facing up side. The EML 71 includes a laser unit consisting essentially of a semiconductor laser diode (LD) and also an optical modulator unit for modulation of the intensity of light emitted from the former. The subcarrier 72 for rigidly supporting this EML chip 71 is made of an electrically insulative material with good thermal conductivity, such as for example aluminum nitride (AlN) ceramics. The subcarrier 72 has its top surface on which several electrodes are provided, which include an electrode for fixation of the EML chip 71, 50-Ohm microstrip line pattern for use in inputting electrical signals to the modulator unit of the EML chip 71, more than one electrode for electrical connection to the thermistor, and electrode(s) for electrical connection to the laser unit of EML chip 71.
In the EML module thus arranged, light emitted from the EML element 71 is collected by the condensing lens 79 and is then optically guided to pass through the glass window 76 attached to the PKG 81 and next penetrate inside of the isolator 87 to finally reach the optical fiber 89 via the coupling lens 88.
Note that the characteristics of the EML element 71 can vary with changes in temperature. In view of this, the EML module is designed so that the thermistor 73 is operable to detect the temperature of EML chip 71 for rendering the Peltier cooler 82 operative by using temperature adjuster circuitry, not shown, to thereby control the temperature of EML element 71.
Also note that transmission signals are sent to the EML element 71 by the 50-Ohm strip line pattern on a microstrip substrate. Unfortunately, certain electrical capacitance can take place between the electrodes formed on the surface of the subcarrier 72 and its associated Peltier cooler. Thus, in order to suppress influence of such capacitance with respect to transmission signals, it is required that the subcarrier 72 be designed to have an increased thickness at a specified value or greater. In practical implementation, transmitting signals at 10 Gbits per second (Gbps) requires the subcarrier 72 to measure in thickness approximately 2 millimeters (mm) or more.
EML modules are such that the outside air temperature is often variable due to the fact that these modules are to be installed for usage under a variety of kinds of environments. In addition, current is injected into the laser unit of the EML element resulting in generation of heat due to its inherent internal resistivity, which in turn causes the element to increase in temperature. The EML element""s characteristics will change as the temperature changes. It is thus required that the EML be used with its temperature made constant during operation thereof. To this end, the EML module is designed so that the thermistor 73 detects the temperature of such EML element to generate a detection signal, which is used to appropriately adjust the cooling/heating performance of Peltier cooler 82 thus stabilizing the temperature so that it stays at a constant level or therearound.
However, as previously stated, the subcarrier 72 must be designed to increase in thickness to measure 2 mm or greater. Additionally, as the subcarrier has on its surface an electrical lead pattern including EML chip connection leads and thermistor connection leads, the subcarrier 72 is a few millimeters in width, which is far greater than the width of EML chip. In addition, the related art subcarrier 72 is made of those materials of good thermal conductivity, such as aluminum nitride (AlN) ceramics.
The above design factors would result in an increase in heat radiation and/or absorption from the top surface and side surfaces of the subcarrier. More specifically, the heat incoming and outgoing from the subcarrier may include not only heat components between the subcarrier and the ELM element but also those as input to and output from the subcarrier""s top and side surfaces. The greater the heat components, the larger the load with respect to the Peltier cooler. One possible approach to avoiding this problem is to employ a high-performance Peltier cooler with increased cooling/heating ability. Regrettably, advantages of the high-performance Peltier cooler do not come without accompanying a penaltyxe2x80x94such cooler is large in size so that the resulting EML module becomes bulky accordingly. The increase in performance can also result in increases in electrical power dissipation of the Peltier cooler. This in turn raises another problem that external circuitry for use in electrically driving or xe2x80x9cfeedingxe2x80x9d such Peltier cooler must also increase in capacity.
The present invention has been made by recognition of presence of the problems faced with the related art. It is therefore a primary object of the invention to provide a subcarrier structure and semiconductor device capable of constantly retaining the temperature of a semiconductor element built therein with increased efficiency by use of a Peltier cooler without reducing or degrading the inherent signal transmission characteristics of a module even upon changing of the temperature of outside environments for usage of the module.
To attain the foregoing object the present invention provides a subcarrier comprising an element support section for immovably supporting a semiconductor element and an element wiring section with more than one electrode for electrical connection to the semiconductor element on its first principal surface, wherein the element support section has its lower part made of a first dielectric material having first thermal conductivity whereas the element wiring section has its lower part made of a second dielectric material having second thermal conductivity less than the first thermal conductivity.
The instant invention also provides a semiconductor device featured by comprising the subcarrier stated above, and a semiconductor element secured to the element support section.
A semiconductor device is also provided which is featured by comprising the subcarrier noted previously, a semiconductor element secured to the element support section, and-a cooler device secured to a second principal surface of the subcarrier opposite to the first principal surface thereof.
Alternatively the invention provides a semiconductor device featured by comprising the subcarrier above, a semiconductor element secured to the element support section, and a cooler device secured to a second principal surface of the subcarrier opposite to the first principal surface thereof, wherein the subcarrier further includes a container section air-tightly enclosing the optical semiconductor element and having a window for permitting transmission of light as emitted from the optical semiconductor element.
More practically, the subcarrier structure incorporating the principles of the invention is a semiconductor element-mounting subcarrier made of an electrically insulative or dielectric material for use in immovably supporting or mounting a semiconductor element with electrodes for electrical connection to the semiconductor element being formed thereon, featured by including an element support section whereat the semiconductor element is rigidly mounted and an element wiring station table which lies around the element support section and on which electrodes are formed, wherein these components are made of materials different in thermal conductivity from each other in a way such that the former is better in thermal conductivity than the latter while causing them to be integrally secured together.
Alternatively, the invention provides a subcarrier with a multilayer structure employing a Peltier cooler supporting the subcarrier with a metal plate sandwiched therebetween, which cooler offers enhanced cooling ability for appropriate incremental/decremental adjustment of the temperature of a semiconductor element used, featured in that a groove is provided between the subcarrier""s element support section and element wiring section, and that contact is made only at limited part between the metal plate and the lower surface of the element support section or between the element support section""s lower surface and the element wiring section""s lower surface portion in close proximity to the element support section while providing a groove between the metal plate and the element wiring section""s lower surface.
Still alternatively the invention provides a subcarrier structure featured in that a metal plate is secured to the lower surface of the aforesaid subcarrier, that a wall made of a dielectric material is provided on a surface on which lead electrodes for electrical connection to an optical semiconductor or optical semiconductor element are formed while causing part of such optical semiconductor element connection lead electrodes to be exposed at outer peripheries of an element wiring section except for a front surface of the optical semiconductor element, that a metallic wall is further provided and rigidly secured to the upper surface of this wall made of the dielectric material and a lateral surface of the element wiring section made of a dielectric material on the light emission side of the optical semiconductor element of an element support section plus the above-noted metal plate without any gaps defined therebetween, that an opening or aperture is formed in the metallic wall on the side of the optical semiconductor element light emission side for permitting outward escape of light emitted from the optical semiconductor element, and that this light-outgoing aperture for use with the optical semiconductor element is associated with either a glass plate or an optical condensing lens being adhered thereto with no gaps therebetween, the lens being for collecting light from the optical semiconductor element.